Various constant flow valves have been proposed in the past. As one, there is the type disclosed in Japanese Unexamined Patent Publication No. 5-99354. In this type of constant flow valve, as shown in FIG. 8, a valve seat 102 provided in a channel 101 and a diaphragm 104 having a valve body 103 facing the valve seat 102 define a diaphragm chamber 105. A force in the direction to open the valve acts on the diaphragm 104 through a spring 106. The diaphragm 104 is formed with a communication passage 107 so that the inlet side fluid flows into the diaphragm chamber 105.
Due to this configuration, the fluid flowing from the inlet side of the constant flow valve into the constant flow valve presses the diaphragm 104 in the to close the valve, is reduced in pressure at the communication passage 107, and then enters the diaphragm chamber 105. The fluid flowing into the diaphragm chamber 105 presses the diaphragm 104 in the direction to open the valve, flows through a fluid control part 108 formed between the valve seat 102 and the valve body 103 of the diaphragm 104 during which it is reduced in pressure, and then flows out to the outlet side of the constant flow valve.
Further, the difference between the closing direction force and the opening direction force acting on the diaphragm 104 is balanced with the spring 106 biasing the diaphragm 104 in the opening direction.
For this reason, if the fluid pressure at the inlet side of the constant flow valve increases or the fluid pressure of the outlet side of the constant flow valve decreases, the closing direction force acting on the diaphragm 104 increases and the opening area of the fluid control part 108 decreases thereby to increase the fluid pressure of the diaphragm chamber 105. Due to this, the opening direction force acting on the diaphragm 104 increases, so that the difference between the closing direction and opening direction forces acting on the diaphragm 104 becomes balanced with the force of the spring 106 again.
On the other hand, if the fluid pressure at the inlet side of the constant flow valve decreases or the fluid pressure at the outlet side of the constant flow valve increases, the opening area of the fluid control part 108 increases, so that the difference between the closing direction and opening direction forces acting on the diaphragm 104 becomes balanced with the force of the spring 106.
Therefore, the difference between the inlet side fluid pressure acting on the diaphragm 104 and the fluid pressure in the diaphragm chamber 105 is held constant, so the pressure difference before and after the communication passage 107 becomes constant and the flow rate can be held constant.
However, since the strength of the spring 106 cannot be changed without disassembling the constant flow valve, the pressure difference before and after the communication passage 107 cannot be changed and therefore the setting of the flow rate cannot be changed after the valve is installed in a pipe.
Further, since the spring 106 contacts the fluid, the metal is liable to dissolve into the fluid or the spring is liable to corrode due to chemicals.
Further, as the valve cannot be completely closed, a separate valve has to be connected in order to cut off the fluid.